Method for manufacturing silver nanoparticles

ABSTRACT

A novel method for manufacturing tabular silver nanoparticles with high productivity and high reproducibility. The present invention provides a method for manufacturing silver nanoparticles including a first step of preparing a silver ion aqueous solution containing a crystal habit modifier as a starting solution, a second step of adding a reducing agent to the starting solution with stirring to yield an aqueous dispersion of fine silver crystals, and a third step of adding an oxidizing agent to the aqueous dispersion with stirring. The oxidizing agent is preferably added such that the solubility of metal silver in the aqueous dispersion is optimized at an appropriate time in the third step.

TECHNICAL FIELD

The present invention relates to a method for manufacturing silver nanoparticles, in particular, a method for manufacturing tabular silver nanoparticles.

BACKGROUND ART

When silver fine particles in the order of nanometers (silver nanoparticles) are exposed to light, free electrons in the particles cooperatively vibrate through the incident light. The resonance between the electric field, which is caused by this vibration of the free electrons, and the incident light (external electric field) generates an enhanced electric field localized on the surface of the particles. This phenomenon is referred to as localized surface plasmon resonance (LSPR). It has been known that silver nanoparticles have an effective extinction (absorption+scattering) cross section that is approximately ten times larger than its physical cross section due to LSPR, providing abnormally strong absorption and scattering of light.

The silver nanoparticles having such light extinction characteristics is expected to be applied to optical materials. Since the wavelength range in which LSPR appears depends on the size of the crystalline particles, a method for manufacturing size-controlled tabular silver nanoparticles with high productivity and high reproducibility has been eagerly desired.

In this regard, Patent Document 1 discloses a method involving a first step of preparing an aqueous solution containing a silver salt, a polycarboxylate, a dispersant, and hydrogen peroxide, and a second step of adding a corresponding amount of reducing agent to the prepared aqueous solution to produce tabular silver nanoparticles with a desired size.

CITATION LIST Patent Document

Patent Document 1: Japanese Translation of PCT Application No. 2008-505252

SUMMARY OF THE INVENTION Problems to be Solved by the Invention

Unfortunately, the silver concentration in the starting solution cannot be increased above 0.1 mM in the method disclosed in Patent Document 1 (hereinafter referred to as the conventional method). Replication study of the conventional method by the present inventors have revealed that the tabular silver nanoparticles based on the conventional method have significantly poor reproducibility of their size and size distribution and is unsuitable for large-scale production, indicating a limited improvement in productivity.

An object of the present invention, which has been accomplished to solve the problem of the conventional method, is to provide a novel method for manufacturing tabular silver nanoparticles with high productivity and high reproducibility.

Means for Solving the Problems

The present inventors have made a hypotheses on the formation mechanism of tabular silver nanoparticles in the conventional scheme (details will be described later). The inventors have revealed problems of the conventional method with reference to the hypotheses and have tried to improve the method. As a result, the inventors have successfully achieved an improvement in productivity (establishment of an increased concentration of final product and an easy scale-up of manufacturing process) and high reproducibility at the same time. The present invention has been thereby accomplished.

According to the present invention, provided is a method for manufacturing silver nanoparticles including: a first step of preparing a silver ion aqueous solution containing a crystal habit modifier as a starting solution; a second step of adding a reducing agent to the starting solution with stirring to yield an aqueous dispersion of fine silver crystals; and a third step of adding an oxidizing agent to the aqueous dispersion with stirring.

Advantageous Effect of the Invention

As described above, the present invention provides a method for manufacturing tabular silver nanoparticles with high productivity and high reproducibility.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram illustrating the steps of manufacturing silver nanoparticles.

FIG. 2 is a conceptual diagram illustrating a selectively growing mechanism of tabular silver nanoparticles.

FIG. 3 shows the measured absorption spectra for aqueous dispersions of silver nanoparticles.

MODES FOR CARRYING OUT THE INVENTION

The present invention will now be described with reference to the embodiments shown in the attached diagrams. The embodiments should not be construed to limit the present invention.

FIG. 1 is a schematic diagram illustrating a method for manufacturing silver nanoparticles according to an embodiment of the present invention. As shown in FIG. 1, the method according to the present embodiment consists of three main steps sequentially explained in the following:

(First Step)

In the first step, a silver ion aqueous solution containing a crystal habit modifier is prepared as a starting solution. Specifically, the silver salt and the crystal habit modifier are added to vigorously stirred water (preferably pure water, more preferably ultrapure water) to prepare the silver ion aqueous solution containing the crystal habit modifier.

The silver salt usable in the present embodiment may be any water soluble compound. A preferred example of the silver salt is silver nitrate (AgNO₃). The productivity of the tabular silver nanoparticles primarily depends on the concentration of silver in the starting solution. In this regard, the concentration of silver in the starting solution can be set at a concentration of 0.2 mM or more, in particular, 0.5 mM or more depending on the target productivity in the present embodiment.

The crystal habit modifier according to the present embodiment may be any compound that can be selectively absorbed on the (111) face of silver crystals. Preferred examples of the crystal habit modifier usable in the present embodiment include low-molecular-weight organic acids and salts thereof. Examples of the low-molecular-weight organic acids include polycarboxylic acids having two or more carboxyl groups. A typical example of the preferred low-molecular-weight polycarboxylic acid is citric acid.

Preferably, the silver salt and the crystal habit modifier should be added in the form of aqueous solutions that are prepared to have appropriate concentrations.

(Second Step)

In the subsequent second step, a reducing agent is added to the starting solution prepared by the above step while the solution is being vigorously stirred. The added reducing agent reduces silver ions in the starting solution to generate very fine silver crystals. The reducing agent in the present embodiment may be any compound that can reduce silver ions to metal silver and has a redox potential appropriately corresponding to the redox potential (+0.799 volt) of silver. Examples of the reducing agent usable in the present embodiment include borohydrides, and a preferred example of such compounds is sodium tetrahydroborate (NaBH₄). Preferably, the reducing agent should be added to the silver ion aqueous solution containing the crystal habit modifier in the form of an aqueous solution of which the concentration is appropriately adjusted at ice temperature.

(Third Step)

In the subsequent third step, while the resulting aqueous dispersion containing fine silver crystals is being vigorously stirred, an oxidizing agent is added to the dispersion. In the present embodiment, the oxidizing agent may be any compound that can oxidize metal silver to silver ions and has a redox potential appropriately corresponding to the redox potential (+0.799 volt) of silver. A preferred example of the oxidizing agent usable in the present embodiment is hydrogen peroxide (H₂O₂) . Preferably, the oxidizing agent should be added to the aqueous dispersion containing fine silver crystals in the form of an aqueous solution of which the concentration is appropriately adjusted.

In the third step, a certain level of silver ions should be constantly supplied to the reaction system over the process to promote Ostwald ripening, which will be described later. Thus, in order to timely optimize the solubility of the metal silver in the aqueous dispersion in the third step, it is preferred that the oxidizing agent is intermittently added in several times, or continuously added in a controlled flow rate while the aqueous dispersion containing fine silver crystals is being vigorously stirred.

A silver colloidal dispersion containing a high concentration of tabular silver nanoparticles as the main component is thereby prepared through the first to third steps described above.

In the present embodiment, the size of the tabular silver nanoparticles in the final product can be controlled by appropriately setting various parameters, such as the concentrations of silver ions and crystal habit modifier in the first step, and the amount of reducing agent to be added, the stirring efficiency, and the reaction temperature in the second step. The reason for optimization of these parameters will be described later. In the present embodiment, the carboxyl groups of the crystal habit modifier is preferably maintained in dissociated forms to keep the pH value of the reaction system at 4 or more over all the steps described above.

As described above, tabular silver nanoparticles can be manufactured with high productivity and high reproducibility in the present embodiment. Since the crystal habit modifier, such as citric acid, used in the present embodiment also functions as a dispersant, no other additional dispersant is required. As a result, the contamination of the final product (the aqueous dispersion containing tabular silver nanoparticles) with any organic compound can be minimized.

The procedure in each step in the method for manufacturing silver nanoparticles according to the present embodiment is described above. The formation mechanism of tabular silver nanoparticles in the present invention will now be described for further understanding.

FIG. 2 is a conceptual diagram for explaining inventors' hypothesis on the mechanism for selectively producing tabular silver nanoparticles. With reference to FIGS. 1 and 2, the formation mechanism of tabular silver nanoparticles will now be described for each step.

In contrast to the conventional method in which polyvinylpyrrolidone (PVP) is required as a dispersant, no polymer compound such as PVP is added to the starting solution in the first step of the present invention, because the used crystal habit modifier also functions as a dispersant.

In the subsequent second step, when the reducing agent is added to the starting solution (the silver ion aqueous solution containing the crystal habit modifier), the silver ions in the aqueous solution are reduced into extremely fine crystal of metal silver with a size of several nanometers, as shown in FIG. 2(a). As shown in FIG. 2(b), at almost the same time as the extremely fine crystals of silver are formed, collisions and associations between the crystals also occur in the aqueous dispersion in a certain probability to form the fine crystals (parallel doubly twinned crystal 10) that can grow into tabular silver nanoparticles. As shown in the enlarged image of FIG. 2(b), the fine silver parallel doubly twined crystal 10 includes two twinning planes (planar defects) which are parallel to each other and also parallel to (111) face of the silver crystal. The principle plane of parallel doubly twined crystal 10 is (111) face. Since the crystal structure of silver is face-centered cubic, from a crystallographic consideration, (100) face is naturally exposed at a part of the side surfaces of parallel doubly twinned crystal 10.

Immediately after the formation of parallel doubly twined crystal 10, crystal habit modifier 12 is adsorbed on the (111) face to inhibit the crystal growth in a direction perpendicular to the (111) face, i.e., the principal plane . Since the amount of crystal habit modifier 12 adsorbed on the (100) face in the side surface is less than that on the (111) face of the principal plane, the inhibition effect of crystal growth on the (100) face is lower than that on the (111) face. As a result, silver seed crystals, which are parallel doubly twinned crystal 10, grow anisotropically, that is, substantially only in the side direction.

In the subsequent third step, addition of the oxidizing agent gives rise to an increase in solubility of metal silver in the aqueous dispersion and thus promotes partial dissolution of fine silver crystals (reionization). Ostwald ripening proceeds under the condition of which the solubility of metal silver is timely optimized. As a result, the larger crystals further grow whereas the smaller crystals further decrease in size.

At the start of the third step, the side (100) face of parallel doubly twined crystal 10 formed in the second step is hardly affected by the adsorption of crystal habit modifier 12; hence, the parallel doubly twined crystal has an advantage in crystal growth rate over the other fine silver crystals contained in the system and thus grow more rapidly compared to the other silver crystals. Once such a difference in size between the parallel doubly twined crystals 10 and the other fine crystals is induced, Ostwald ripening is accelerated according to its principle as long as the solubility of metal silver is maintained. As shown in FIG. 2(c), the tabular crystals arising from parallel doubly twined crystal 10 having larger size grow preferentially. As a result, tabular silver nanoparticles 20 with an increased diameter in the principal plane are manufactured as a main component as shown in FIG. 2(d).

In the late stage of the third step, the Ostwald ripening among tabular silver nanoparticles 20 having different sizes further proceeds to so-called over-ripening. Consequently, tabular silver nanoparticles 20 having a uniform size are obtained as a final product in a proportion of almost 100% in number.

The formation mechanism of the tabular silver nanoparticles in the present invention is explained above. In this mechanism, the primary factor to determine the size of tabular silver nanoparticles 20 in the final product is the number ratio of the parallel doubly twined crystals 10 to all of the other fine crystals at the end of the second step. In other words, higher number ratio of parallel doubly twinned crystals 10 results in smaller size of tabular silver nanoparticles 20 in the final product; lower number ratio of parallel doubly twinned crystals 10 results in larger size of tabular silver nanoparticles 20 in the final product.

The number ratio of the parallel doubly twined crystals 10 to all of the other fine crystals at the end of the second step alters within a range of several percent to several tens of percent, which depends on various parameters, such as the concentrations of the silver ions and the crystal habit modifier in the first step, the molar ratio of the silver ions to the crystal habit modifier in the first step, and the amount of the reducing agent to be added, the reaction temperature, and the stirring efficiency in the second step. In other words, the optimization of these parameters can control the number ratio of parallel doubly twined crystals 10, and thus can obtain tabular silver nanoparticles with a target size.

The formation mechanism (hypothesis) of the tabular silver nanoparticles in the present invention is explained above. In view of this hypothesis, the disadvantages in the conventional method may be explained as follows.

First, polyvinylpyrrolidone (PVP) added as a dispersant to the starting solution in the conventional method is likely to be adsorbed on the (100) face of silver crystal. This adsorption gives rise to an increase in thickness of tabular silver nanoparticles, and also inhibits the Ostwald ripening in the third step.

Second, in the conventional method, citric acid coexists with hydrogen peroxide in the starting solution. In that situation, citric acid is gradually oxidized by hydrogen peroxide before the reduction of silver ions and is not likely to function as a crystal habit modifier.

Third, in the conventional method, reducing agent (sodium tetrahydroborate) in the reaction system coexists with oxidizing agent (hydrogen peroxide) in the step to form seed crystals (parallel doubly twinned crystals) by the reduction of silver ions, causing the deactivation of reducing agent by oxidizing agent. As a result, the reduction of silver ions in the reaction system is so unstable that it is very difficult to control the formation of parallel doubly twinned crystals which can grow into tabular silver nanoparticles.

Fourth, in the conventional method, hydrogen peroxide (oxidizing agent) is added only to the starting solution. Furthermore, since hydrogen peroxide is substantially used by the oxidization of organic compounds (citric acid and PVP) contained in the reaction system before the Ostwald ripening step described above, it is quite difficult to proceed Ostwald ripening into over-ripening.

The problems in the conventional method are explained above. The present inventors reconstructed the conventional method in view of those problems to be solved, and have successfully manufactured tabular silver nanoparticles with higher concentration and higher reproducibility than ever before. At the same time, it has been found that the reconstructed manufacturing method achieves the easiness for scale-up. The present invention is thus completed.

The method for manufacturing tabular silver nanoparticles of the present invention is described above. Their known applications include chemical reagents (in particular, diagnostic agents (biosensors) and sensitizers for spectroscopy, such as surface enhanced Raman scattering), paints, antistatic films, electroconductive films, antireflective films, antibacterial films, and catalyst-supporting films. Since thin layer containing tabular silver nanoparticles can trap light, the efficiency of photoelectric conversion in the devices such as solar cells may be improved by the coating of tabular silver nanoparticles on their photo-accessible surface. Specifically, the tabular silver nanoparticles whose LSPR is tuned to the light absorption band of a dye and a p-type organic semiconductor are included in a dye sensitized solar cell and an organic thin film solar cell, respectively. The enhanced electric field arising from the LSPR of these tabular silver nanoparticles may improve their efficiency of photoelectric conversion.

The present invention is described by the embodiment on a method for manufacturing silver nanoparticles above. Besides silver, the present invention can also be applied to noble metals, such as copper, gold, platinum, palladium, and rhodium, to manufacture tabular nanoparticles of these metals. Other materials that have advantageous effects of the present invention are also included in the scope of the present invention within the embodiments conceivable by a person skilled in the art.

EXAMPLES

The method for manufacturing silver nanoparticles of the present invention will now be described in further details by way of examples. These examples should not be construed to limit the present invention.

<Preparation of Aqueous Dispersion of Silver Nanoparticles>

Silver nanoparticles were prepared according to the following procedures. All the reagents were special grade reagents available from Wako Pure Chemical Industries, Ltd.

While ultrapure water (160 ml) was being stirred, an aqueous solution of 500 mM trisodium citrate (3.38 ml) and an aqueous solution of 500 mM silver nitrate (225 μl) were added to the ultrapure water in sequence to prepare the starting solution (silver concentration: 0.68 mM). While the resulting starting solution was being stirred, an aqueous solution of 600 mM sodium tetrahydroborate (1.13 ml) prepared at ice temperature was added as a reducing agent to the starting solution. A pale yellow aqueous dispersion of silver nanoparticles was thereby prepared (hereinafter, the aqueous dispersion is referred to as Sample 1).

The same aqueous dispersion as Sample 1 was prepared, 30% aqueous hydrogen peroxide (5.4 ml) was added thereto, and then the dispersion was stirred for one hour. A dark brown aqueous dispersion of silver nanoparticles was thereby prepared (hereinafter, the aqueous dispersion is referred to as Sample 2).

The same aqueous dispersion as Sample 1 was prepared, and the manufacturing process including addition of 30% aqueous hydrogen peroxide (5.4 ml) and stirring of the dispersion for one hour was carried out twice. A violet aqueous dispersion of silver nanoparticles was thereby prepared (hereinafter, the aqueous dispersion is referred to as Sample 3).

The same aqueous dispersion as Sample 1 was prepared, and the manufacturing process including addition of 30% aqueous hydrogen peroxide (5.4 ml) and stirring of the dispersion for one hour was carried out three times. A indigo aqueous dispersion of silver nanoparticles was thereby prepared (hereinafter, the aqueous dispersion is referred to as Sample 4).

<Measurement of Absorption Spectra of the Aqueous Dispersions of Silver Nanoparticles>

The absorption spectra of Samples 1 to 4 prepared in the procedure described above were measured with V-570UV/Vis/NIR spectrometer (JASCO Corporation). In the measurement, light path length of cell was 2 mm, each sample was used without dilution, and ultrapure water was used as a reference. FIG. 3 shows the measured absorption spectra of Samples 1 to 4.

The spectrum of Sample 1 includes two absorption bands at approximately 345 nm and 490 nm assigned to tabular silver nanoparticles, and a single absorption band at approximately 420 nm assigned to non-tabular silver nanoparticles. This spectrum indicates that Sample 1 contains both tabular and non-tabular silver nanoparticles.

The spectrum of Sample 2 has an absorption maximum at approximately 420 nm, which is smaller than that of Sample 1. This comparison indicates that the non-tabular silver nanoparticles in Sample 2 are less in number than those in Sample 1. The absorption band at approximately 490 nm in Sample 1 is shifted to approximately 650 nm in Sample 2 with increased absorption. This comparison indicates that the major diameter of the tabular silver nanoparticles in Sample 2 is larger than that in Sample 1.

The spectrum of Sample 3 has an absorption maximum at approximately 420 nm that is smaller than that of Sample 2. This comparison indicates that the non-tabular silver nanoparticles in Sample 3 is less in number than those in Sample 2. The absorption band at approximately 650 nm in Sample 2 is shifted to approximately 720 nm in Sample 3 with increased absorption. This comparison indicates that the major diameter of the tabular silver nanoparticles in Sample 3 is larger than that in Sample 2.

The spectrum of Sample 4 has no absorption band at approximately 420 nm. The disappearance indicates that Sample 4 no longer contains non-tabular silver nanoparticles. The absorption band at approximately 720 nm in Sample 3 is shifted to approximately 750 nm in Sample 4 with increased absorption. This comparison indicates that the major diameter of the tabular silver nanoparticles in Sample 4 is larger than that in Sample 3 and that Sample 4 contains only tabular silver nanoparticles with a sharp size distribution.

In Sample 4, the absorption maximum at 750 nm was “1.35”, when determined with light path length of 2 mm. This corresponds to “6.75” for that of 1 cm. This absorbance is equivalent to approximately seven times compared to that of a colloidal dispersion primarily composed of tabular silver nanoparticles prepared by the conventional method. No report has been disclosed on the preparation of a colloidal dispersion primarily composed of such a high concentration of tabular silver nanoparticles.

Repeated manufacturing under the same conditions as above provided high reproducibility on the concentration and size distribution of tabular silver nanoparticles contained in the final colloidal dispersion (within ±10 nm in absorption maximum).

Ten-fold scale-up manufacturing under the same conditions also provided high reproducibility on the concentration and size distribution of tabular silver nanoparticles contained in the final colloidal dispersion (the volume was 2.5 litters). The results imply that the manufacturing method of the present invention is industrially useful.

REFERENCE SIGNS LIST

10 parallel doubly twined crystal

12 crystal habit modifier

20 silver nanoparticles 

1. A method for manufacturing silver nanoparticles comprising: a first step of preparing a silver ion aqueous solution containing a crystal habit modifier as a starting solution; a second step of adding a reducing agent to the starting solution with stirring to yield an aqueous dispersion of silver crystals; and a third step of adding an oxidizing agent to the aqueous dispersion with stirring.
 2. The method of claim 1, wherein, in the third step, the oxidizing agent is added such that the solubility of metal silver in the aqueous dispersion is timely optimized.
 3. The method of claim 1, wherein the crystal habit modifier comprises a low-molecular-weight organic acid or a salt thereof.
 4. The method of claim 3, wherein the low-molecular-weight organic acid is a polycarboxylic acid having two or more carboxyl groups.
 5. The method of claim 4, wherein the polycarboxylic acid is citric acid.
 6. The method of claim 1, wherein the reducing agent is sodium tetrahydroborate.
 7. The method of claim 1, wherein the oxidizing agent is hydrogen peroxide.
 8. The method of claim 1, wherein, in the third step, tabular silver nanoparticles are selectively manufactured in the water aqueous dispersion.
 9. Silver nanoparticles manufactured by the method of claim
 1. 10. A colloidal dispersion of the silver nanoparticles of claim
 9. 